Wave Ducting in a Stratified Shear Flow over a Two-Dimensional Mountain. Part II: Implications for the Development of High-Drag States for Severe Downslope Windstorms

A numerical study on severe downslope windstorms occurred on 5 April 2005 at Gangneung and Yangyang, Korea

“…13 satisfies these conditions of the hydraulic-jump mechanism. The features of the strong downslope wind in P3 are analogous to the windstorm case on 11 January 1972 at Boulder, Colorado, which was investigated in many previous studies (Hoinka, 1985;Durran, 1986;Wang and Lin, 1999).…”

The physics of orographic gravity wave drag

“…This implies that waves with a wavenumber vector perpendicular to the mean flow (k = 0 and l = 0) are totally absorbed, no matter how low Ri c is, whereas the attenuation of waves with wavenumber vectors parallel to the mean flow (l = 0) is similar to that of 2D waves. Following Lindzen and Tung [75], but carrying out a more systematic exploration of parameter space, [76,77] used linear theory to address wave ducting, and its implications for downslope windstorms, in flow over 2D mountains. They treated cases where both Ri c > 0.25, and thus the critical level absorbs upward propagating waves in accordance with (47), and cases where Ri c < 0.25, where the waves are unattenuated, but instead amplified by the critical level, in what has been termed "overreflection" [75].…”

“…Using a piecewise-linear wind profile qualitatively similar to (55), Wang and Lin [76,77] studied the implications of wave ducting for downslope windstorms and high-drag states. They found that, at least in the linear regime, wave reflections at the shear discontinuities existing below and above the critical level (whose intensity is controlled by Ri c ) rather than at the critical level itself determine the wave response, with wind and drag maxima at the surface attained when (58) is fulfilled, where now z 1 is the height of the lowest shear discontinuity.…”

“…They explained some of the discrepancies observed between previous studies on highdrag states (namely [73] and [92]) by the use of z c instead of z 1 as a key flow parameter. Wang and Lin [77] also showed how nonlinear high-drag states (with a spacing of λ z rather than λ z /2 as a function of z 1 ) are selected from the linear ducted wave modes through modification of the static stability profile by the wave flow perturbations.…”

A review of recent advances in understanding the meso- and microscale properties of the severe Bora wind

“…Another model stresses the role of non‐linear wave‐ducting mechanism prior to the hydraulic jump formation (e.g. Wang and Lin, 1999). The corresponding early sequence of detailed events toward establishing severe bora state, for example, the upwind boundary layer evolution, has not been well documented yet.…”